1. Field of the Invention
The present invention relates to a torque limiting device of a reel turntable for use with a video tape recorder (VTR) or the like, for example.
2. Description of the Prior Art
Conventional reel turntable torque limiting mechanisms or devices employ a torque limiting device using a felt mat or a magnet.
FIG. 1 of the accompanying drawings is a cross-sectional view showing a reel turntable torque limiting mechanism using a magnet. A reel turntable mechanism that is generally designated by reference numeral 1 will be described initially. As shown in FIG. 1, the reel turntable mechanism 1 comprises a disk-shaped rotary member 3 integrally attached to an intermediate portion of a cylindrical-shaped rotary shaft 2 so as to extend outwardly, a cap portion 4 engaged into the upper portion of the rotary shaft 2 with a pressure, a reel turntable 5 that rotates in unison with the rotary shaft 2, a reel member 6 inserted into the upper end portion of the cap portion 4 so as to become freely slidable in the axial direction of the rotary shaft 2 and a coil spring 7 interposed between the reel member 6 and the reel turntable 5 for biasing the reel member 6 upwardly under spring force.
The reel turntable device 1 thus arranged is designed such that the rotary shaft 2 is inserted into a shaft 9 vertically erected from a chassis 8 to freely rotate. The reel turntable device 1 is rotated by a torque limiting device 10.
The torque limiting device 10 will be described below. The above-mentioned rotary member 3 has on its rear surface a disk-shaped yoke member 11 supported by an insert molding process. The yoke portion 11 has on its rear surface an annular-shaped magnet 12 made of, for example, ferrite attached. More specifically, the magnet 12 is engaged with a protruded portion 3a formed on the rotary member 3, for being prevented from being rotated relative to the rotary member 3. Also, the magnet 12 is fixed to the rotary member 3 by an adhesive, not shown.
A limiting gear 13 is rotatably supported at its shaft portion 13b to the rotary shaft 2 projected from the lower side of the rotary member 3. The limiting gear 13 has a magnetic plate 14 unitarily molded thereto by a so-called outsert molding process. The magnetic plate 14 is located near the above-mentioned magnet 12 to oppose the same, and the limiting gear 13 has on its outer peripheral portion a gear portion 13a. A spacing between the magnet 12 and the magnetic plate 14 is kept by a washer 15 made of a non-magnetic material interposed between the magnet 12 and the magnetic plate 14. The washer 15 is served also as a thrust block for the limiting gear 13.
The reel turntable device 1 is rotated by the torque limiting device 10 as follows.
When the limiting gear 13 is rotated by the drive gear (not shown), the magnet 12 generates a limiting torque in accordance with the rotation of the magnetic plate 14 because the magnetic plate 14 is magnetically attracted by the magnet 12 to thereby rotate the rotary member 3 about the shaft 9. Then, the reel turntable 5 is rotated in unison with the rotation of the rotary member 3. When the reel turntable 5 is overloaded by an excess tension applied to a cassette tape (not shown), the magnet 12 slips relative to the magnetic plate 14 so that the reel turntable 5 together with the rotary member 3 removes an excess tension applied to the tape.
The conventional torque limiting mechanism thus arranged generates a limiting torque by a magnetic attraction force generated between the magnet 12 and the magnetic plate 14. Simultaneously, a friction is generated among the magnetic plate 14, the magnet 12 and the washer 15 by a magnetic attraction force between the magnet 12 and the magnetic plate 14. The friction generated by the washer 15 is changed with the change of ambient temperature, the change of humidity or the change of speed of the limiting gear 13 or the like, thereby a limiting torque being fluctuated. As a consequence, such fluctuation of limiting torque exerts a bad influence upon the tape travel so that the tape is damaged.